Electrophotographic recording apparatus

ABSTRACT

An electrophotographic recording apparatus comprising these units which are arranged vertically: a sheet feeder for feeding a sheet of recording paper to a sheet feed inlet; an image forming unit for forming a printing image onto a photosensitive drum based on printing data and transferring the printing image onto the sheet; and a fixing unit for fixing the transferred image on the sheet. The apparatus further comprises a sheet switching guide mechanism by which the head end of the sheet is switched to both upper and lower directions. When the sheet switching guide mechanism is set to the upper direction, the printed sheet is forwarded by a sheet guide member to thereby pass through the printing path the sheet has already been through with the rear side of the sheet being the front, and the printing data is written from the last line during this process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrophotographic recording apparatuswhich prints characters and graphics on a sheet of paper usingelectrophotographic technology, and more particularly to a technique ofhandling the recording paper from the paper stocking position to theprinted paper collecting position.

2. Description of the Related Art

In attempts to respond to the needs for high-quality and high-speedprinting, electrophotographic recording apparatuses based on theprinciple of the so-called "xerography" have been developed forpractical applications. "Xerography" involves the process in which theoptical beam controlled by printing data is irradiated onto acylindrical photosensitive body to form a latent image thereon; coloredtoner is caused to adhere electrostatically to the latent image thusformed; and the toner image is then transferred to recording paper tothereby subject the image to a thermocompression bonding.

The printing technique using these electrophotographic recordingapparatuses can produce high-density images at high printing speeds perunit time, thereby replacing conventional printing techniques such asoffset printing and typography with the so-called desktop publishing inthe field of small-scale printing.

In order to provide the best service in such a field of printing,electrophotographic recording apparatuses must be equipped withfunctions of printing on both sides of a sheet, sorting the printedsheets by page, printing large amounts of sheets with one supply ofsheets, and requiring only a small floor area of installation.

In order to meet these requirements, Japanese Patent Application (UPA)Nos. 167853/1989, 170953/1989 (the term "UPA" as used herein means"Unexamined Published Application") disclose electrophotographicrecording apparatuses whose printing path is arranged vertically whichcauses a printed sheet to be inverted.

These electrophotographic recording apparatuses, allow for the requiredarea of installation to be reduced in proportion to the realizedreduction in sheet forwarding floor area.

However, such electrophotographic recording apparatuses have notaddressed the problem of sorting the printed sheets by page. The printedsheets are only piled up in the same tray when a large quantity ofcopies are made.

In addition, these electrophotographic recording apparatuses areavailable only for cut sheet printing and may not be capable ofproducing high-quality printing for continuous forms and slips in whichpositioned entries are required because they may print out of thespecified position.

The arrangement of a stacker or vertical stackers in order to reduce thefloor area of installation not only causes sheets to be bent due tosheets falling from the sharply standing stacker, but also entails thecumbersome operation of attaching and detaching the stacker to theapparatus body.

In order to overcome these problems, Japanese Patent Application (UPA)Nos. 76073/1989 and 49059/1989 disclose a type of electrophotographicrecording apparatus with its stacker(s) arranged horizontally.

However, in such an electrophotographic recording apparatus, the endportion of the cassette containing the recording sheets is locatedfurther behind the recording mechanism, and as a result the cassetteextrudes from the apparatus body, thereby requiring a larger floor areaof installation.

SUMMARY OF THE INVENTION

A first object of the present invention is therefore to provide a novelelectrophotographic recording apparatus capable of continuously printingboth front and rear sides of a recording sheet without increasing thefloor area of installation.

A second object of the present invention is to provide an releasableelectrophotographic recording apparatus capable of arranging a pagesorter for printed sheets without increasing the floor area ofinstallation.

A third object of the present invention is to provide a novelelectrophotographic recording apparatus capable of selectively printingon both a cut sheet and a continuous sheet.

A fourth object of the present invention is to provide a novelelectrophotographic recording apparatus capable of continuously printinglarge amounts of copies with one supply of recording sheets.

A fifth object of the present invention is to provide anelectrophotographic recording apparatus capable of properly loading arecording sheet by horizontally stacking the sheet without increasingthe floor area of installation.

Other objects of the present invention will be apparent from thedescriptions of the embodiments thereof.

In order to achieve these objects, the electrophotographic recordingapparatus according to the present invention includes means for feedingsheets, means for forming a printing image onto a photosensitive drumbased on printing data and transferring the printing image onto thesheet, and means for fixing the transferred image on the sheet. Thesemeans are arranged vertically in upper and lower portions. In additionsheet switching guide means for switching the head end of the sheet intwo directions is arranged at a sheet discharge outlet of the fixingmeans. When the sheet switching guide means is set to the upperdirection, a printed sheet is forwarded to a second sheet guide member,inverted, and passed through the original printing path, in whichprocess the printing data is written from the last line on the sheet.Accordingly, the required floor area of installation of the apparatusmay be reduced as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the general configuration of asystem using an electrophotographic recording apparatus according to thepresent invention;

FIG. 2 is a sectional view showing a first embodiment of the systemaccording to the present invention in which the electrophotographicrecording apparatus, a sorter, and a large-capacity sheet feeder arecombined;

FIGS. 3 and 4 are sectional views showing the structure of a secondsheet guide member of the apparatus shown in FIG. 2, and a front view ofone of the board members, respectively;

FIG. 5 is a sectional view showing the structure of a sheet separatingmember of the apparatus shown in FIG. 2 and its vicinity;

FIG. 6 is a block diagram showing a control unit of the apparatus shownin FIG. 2;

FIGS. 7(A) through 7(E) are sequential diagrams explaining an operationof the apparatus shown in FIG. 2;

FIGS. 8A and 8B are diagrams showing examples of front-side printing andback-side printing by the apparatus shown in FIG. 2, respectively;

FIG. 9 is a flowchart showing an operation of the apparatus shown inFIG. 2 in which one-side printing and double-side printing are performedfor a single copy;

FIG. 10 is a flowchart showing an operation of the apparatus shown inFIG. 2 in which one-side printing and double-side printing arecontinuously performed for a plurality of copies;

FIG. 11 is a schematic showing the appearance of a second embodiment ofthe electrophotographic recording apparatus according to the presentinvention;

FIG. 12 is a sectional view showing the structure of the apparatus shownin FIG. 11;

FIGS. 13 through 15 are diagrams showing how a paper jam is corrected atvarious stages in the printing process for the apparatus shown in FIG.11, respectively;

FIG. 16 is a sectional view showing a third embodiment of theelectrophotographic recording apparatus according to the presentinvention;

FIG. 17 is an enlarged sectional view showing the structure and locationof a first and a second tractor feeder in the apparatus shown in FIG.16;

FIG. 18 is a sectional view showing the state of cutting the sheet inthe apparatus shown in FIG. 16;

FIG. 19 is a perspective view showing a fourth embodiment of theelectrophotographic recording apparatus according to the presentinvention; and

FIG. 20 is a sectional view showing the structure of the apparatus shownin FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a configuration of an electrophotographic recordingapparatus according to the present invention. The electrophotographicrecording apparatus, reference numeral 2 is very often used installednear a personal computer system 10 which includes a computer body 4, adisplay unit 6, and a keyboard 8. The operation panel 12 on such anelectrophotographic recording apparatus 2 is oriented substantially thesame as the keyboard 8. The stackers 14 and 16, used stacking sheets ofrecording paper, are releasable and mounted on the upper portion of theapparatus for paper feeding into the apparatus. The manual insertion ofone sheet of paper possible with P is the stacker located closest to theoperator. The printed paper is discharged with the printed side facedownto a tray 18 located in front of the apparatus.

FIG. 2 shows a first embodiment of the electrophotographic recordingapparatus. Reference numeral 20 designates a first sheet feederreleasably mounted on the upper portion of the apparatus body 22. Theapparatus in this embodiment is provided with stackers 24 and 26allowing two different sizes of printing paper P1 and P2 to be thereinstocked. Pickup rollers 28 and 30 are for taking these sheets of paperP1 and P2 out of the stackers on a sheet by sheet basis and thereby feedthrough sheet discharge outlets 32 and 34 to a sheet feed inlet 36.

On the lower portion of the sheet feed inlet 36 is a sheet detector 38for detecting the setup of a sheet of recording paper to be printed.When the head end of the sheet is properly positioned in a predeterminedmanner, the sheet is forwarded to gate rollers 40 whereby the sheet istaken down to a printing mechanism.

Downstream of the gate rollers is the printing mechanism beginning witha photosensitive drum 44 (which rotates in the direction of arrow A inorder to forward the recording paper downward) and an optical beam froman optical beam control mechanism 42 which is used for writing.Positioned around the photosensitive drum 44 are: a charging unit 46, adeveloping unit 48, a sleeve 50, a transfer unit 52, a toner recoveringunit 54, and an eraser 56. Below the photosensitive drum 44 is a fixingunit 58 for fixing the toner transferred on the recording paper. Allthese components constitute the printing mechanism.

Below the fixing unit 58, (i.e., on the side of a sheet dischargeoutlet) are fixed sheet discharge guide boards 60 and switching guideboards 64 that turn around the sheet discharge outlet 62. The switchingguide boards 64 serve to change the path of the sheet. When turned inthe direction of the arrow B, the sheet is forwarded to dischargerollers 66; whereas, when turned in the direction of the arrow C, thesheet is directed to a second sheet guide 68.

The second sheet guide member 68 comprises a sheet guide board 72, whichextends from the switching guide boards 64 to the uppermost sheettake-out rollers 70, and includes therebetween sheet forwarding rollers74 and 76. The pairs of sheet forwarding rollers, 74 and 76, are spacedshorter than the minimum sheet length to facilitate forwarding the sheetupward. This second sheet guide member 68 is arranged on the front sideof the apparatus body 22 (i.e., on the side of the sheet forwardingpath). The sheet take-out rollers 70 take the sheet up from the sheetguide board 72 and then are reversely rotated by a signal from a tailend detector 78, and thereby, forward the sheet P back downward.

The tail end detector 78, used for detecting the tail end of the sheetwhen rear-side printing is required, is located below the take-outrollers 70 and above the top of a triangular sheet forwarding pathswitching member 80.

FIGS. 3 and 4 show an embodiment of the sheet guide board 72, whichcomprises board bodies 90 and 91 confronting each other at apredetermined distance. The confronting surface of each of the boardbodies, 90 and 91, is provided with a plurality of projecting members 92and 93 in parallel with each other in the sheet forwarding direction.Projecting members 92 confront projecting member 93 in a staggeredmanner.

FIG. 5 shows the structure of the sheet forwarding path switching member80 and its location. The top of this sheet forwarding path switchingmember 80 has a vertical cross-section which is triangular facing thetake-out rollers 70 and is cut so as to expand the space or the entranceof an upward printing path 96, upward. The top 95 is located outside thetangent T of the take-out rollers 70, (i.e., to the left of the figure)so that the tail end of the sheet can easily be received from an upwardto a downward printing path 96. On a slope 97 of the sheet forwardingpath switching member 80 is a guide board 98 that extends from its endportion and confronts the member 80 at an interval allowing the sheet tobe guided. On the other slope 99 is a guide board 100 that is arrangedto form the printing path 96. This guide board 100 is arranged so thatit intersects the tangent T at a pressure contact point of the take-outrollers 70 at an angle θ, which is approximately 5°, and the head end ofthe sheet pulled down from the rollers 70 is caused to come into contactwith the guide board 100.

In FIG. 2, reference numeral 101 designates a sorter that is releasablymounted to the electrophotographic recording apparatus. The sorter hassorter forwarding rollers 102 at a position where they are on the samelevel as the discharge rollers 66 of the electrophotographic recordingapparatus body. On the end portions of the sorter forwarding rollers 102(i.e., on the discharge side) there are sorter guide boards 103 thatrise upright from the horizontal surface. On the vertical surface are aplurality of bin guides 104, 105, 106, 107, and 108 that are arranged ata predetermined interval and bin selecting gate members 109, 110, 111,and 112 that serve to send the sheet into the bin guides. The endportions of the bin guides 104 through 108 are provided with sheetdischarge rollers 114, 116, 118, 120, and 122, respectively. The endportions of the sheet discharge rollers 114 through 122 are providedwith releasable bin trays 124, 126, 128, 130, and 132 so that eachdischarge outlet faces downward.

Also in FIG. 2, reference numeral 140 designates a large-capacity sheetfeeder, which is releasably mounted to the electrophotographic recordingapparatus body 22, for continuously supplying a large quantity ofpre-stacked sheets. Reference numeral 142 designates a large-capacitycassette capable of horizontally stocking some 2000 sheets of recordingpaper. The bottom of this cassette is equipped with a push-up mechanism144 for successively feeding the discharge outlet pickup roller 146according to the number of sheets printed. Whereas, the top of thecassette 142 is provided with the discharge outlet pickup roller 146 anda pickup pad 148 which each come into contact with theelectrophotographic recording apparatus.

FIG. 6 shows an embodiment of a control unit, in which reference numeral150 designates a microcomputer including a central processing unit (CPU)152, a random access memory (RAM) 154, and a read only memory (ROM) 156.The microcomputer 150 controls not only the optical beam controlmechanism 42 (FIG. 2) based on printing data transmitted from the hostunit 10 (FIG. 1), but also the sheet forwarding path based on a signalfrom the respective detectors 38 and 78 depending on the single-sideprinting mode or double-side printing mode set via the operation panel12 or the host unit 10 (FIG. 1).

Reference numeral 158 designates a main buffer memory for receiving theprinting data from the host unit 10. This buffer memory 158 stores theprinting data in response to a command from the microcomputer 150 andapplies the printing data to the optical beam control mechanism 42through the microcomputer 150 at the time of printing.

Reference numeral 160 designates a double-printing buffer memory whichreads the printing data out of the main buffer memory 158 from the lastline, stores it, and applies it to the optical beam write controlmechanism 42.

An operation of the electrophotographic recording apparatus embodimentthus constructed will next be described with reference to theexplanatory diagrams shown in FIG. 7.

When the photosensitive drum 44 is rotated at a constant speed, itssurface is charged to a predetermined potential by the charging unit 46.When a light beam is irradiated onto the photosensitive drum 44 surfaceaccording to the image data from the optical beam control unit 42, asdescribed above, the irradiated portion becomes electroconductive andforms a latent image on the photosensitive drum 44. This latent image isrendered into a real image or a toner image by its contact with thetoner through the sleeve 50 of the developing unit 48.

Single-side printing mode:

Upon instruction of the single-side printing mode via the host unit orthe operation panel, the microcomputer 150 sets a path for a directoutput of the main buffer memory 158 to the optical beam write controlmechanism 42, bypassing the double-side printing buffer memory 160. Uponoutput of printing data from the host unit, the microcomputer 150 notonly causes the main buffer memory 158 to store that data (Step 501 inFIG. 9), but also prepares the electrophotographic recording apparatusfor printing.

That is, the sheet P stocked in the stacker section 24 or 26 in thefirst sheet feeder 20 is introduced to the sheet feed inlet 36 from thepickup rollers 28 or 30; the head end of the sheet P is stopped by thegate rollers 40. The sheet P is then further forwarded so that it willbe bent, thereby causing the sheet detector 38 to operate (Step 502). Asignal from the sheet detector 38 causes gate rollers 40 to rotatesynchronously with the rotation of the photosensitive drum 44 (thesurface of which has adhered toner); thereby the sheet P is forwardedtoward this photosensitive drum 44. The transfer unit 52 electrifies theback of the sheet P and attracts the toner that is on the surface of thephotosensitive drum 44 toward the surface of the sheet P, and therebythe toner is transferred onto the sheet. The transferred toner is fixedon the sheet P while the sheet is passing through the fixing unit 58(Steps 503 and 504).

The sheet P that has exited from the fixing unit 58 is discharged to thedischarge roller 66 while being guided by the fixed sheet dischargeguide boards 60 and the switching guide boards 64 (FIG. 7(A)) (Step505). If printing of a successive page is required (Step 506), thatprinting data is stored in the main buffer memory 158 (Step 507) and theprinting operation beginning with Step 502 and on is repeated for thedesired number of pages.

DOUBLE-SIDE PRINTING MODE

Upon instruction of the double side printing mode (Step 500), themicrocomputer 150 first sets a path for a direct output of the mainbuffer memory 158 to the optical beam write control mechanism 42. Uponreceipt of the printing data from the host unit, the microcomputer 150stores it in the main buffer memory 158 (Step 508).

The above operation is for single-page printing. In order to start witha second page with proper pagination (Step 509), the microcomputer 150causes a first sheet P stocked in the stacker section 24 or 26 to beushered into the sheet feed inlet 36 by the pickup rollers 28 or 30; thehead end of the sheet P stopped by the gate rollers 40. The and thesheet P is then further forwarded to thereby operate the sheet detector38 (Step 510). At this point, the microcomputer 150 not only causes thegate rollers 40 to operate, but it also causes the optical beam writecontrol mechanism 42 to write the printing data within the main buffermemory 158 to the photosensitive drum 44. The photosensitive drum 44rotates synchronously with the forwarding speed of the sheet P adjustedby the gate rollers 40. The latent image is thereby formed then renderedinto a real image by the developing unit 48; this real image istransferred onto one of the sides from head to tail of the sheet P whileentering the transfer region of the transfer unit (FIG. 8A). Then thesheet enters the fixing unit 58 and has the toner applied to its surfacein order to be fixed; it is and then further forwarded to the fixedsheet discharge guide boards 60 (Step 511).

Upon completion of the writing of the printing data of the second pageonto the photosensitive drum 44 (Step 512), the microcomputer 150requests the host unit to transmit the printing data of the first page(i.e., the front side, Step 513), and stores this data in the mainbuffer memory 158 (Step 514).

The microcomputer 150 switches the switching guide boards 64 to thesecond guide member 68 side (Step 516). This causes the sheet to beforwarded upward (FIG. 7(B) while guided by the second guide member 68with the second guide rollers 74, thereby bypassing the dischargerollers 66. of the sheet moves upward along the sheet guide board 72,the projecting members 92, 92, 92 . . . and 93, 93, 93 . . . , whichextend vertically, help cause the sheet to go up along the second guidemember smoothly with only minimal friction. Since the projecting memberson the one side and those on the other side are staggered, the sheetgets slightly, so to speak, "corrugated" and thus becomes firm. For thisreason, even if the head end of the sheet touches an obstacle, it canovercome the obstacle without being folded, thereby moving upwardwithout being jammed.

In this way, the sheet P is guided up to the sheet take-out rollers 70(FIG. 7(C)) and when the tail end of the sheet P passes by the tail enddetector 78, it produces a signal to reversely rotate the take-outrollers 70 (FIG. 7(D). As a result, the sheet P is forwarded downward.

In the process of drawing the sheet downward, the tangent of thetake-out rollers 70 intersects the surface of the guide board 100, sothat the tail end of the sheet is forwarded downward in contact with theguide board 100. Furthermore, the upper end of the forwarding pathswitching member 80 allows the space above it to be open wider ensuringthat the sheet is guided to the printing path side, namely the sheetfeed inlet 36.

Accordingly, the blank surface of the sheet faces the photosensitivedrum 44 which is set with a predetermined width (i.e., with a bindingmargin L that corresponds to that of the front page) (FIG. 7(E)). Atthis stage, the take-out rollers 70 are rotated so as to forward thesheet P downward. As a result, the sheet P is bent with its tail endpressed by the gate rollers 40 to thereby operate the sheet detector 38(Step 517). Upon storing of the first page data in the main buffermemory 158, the microcomputer 150 starts reading this data from thestart of the last line and writes to the double-side printing buffermemory 160.

The microcomputer 150 then rotates the gate rollers 40 synchronouslywith the writing to the photosensitive drum 44 and forwards the sheetfrom the side which will be the last line of printing to thephotosensitive drum 44. Then the data is read from the first address ofthe double-side printing buffer memory 160 and applied to the opticalbeam write control mechanism 42. This causes the printing data to beprinted on the sheet P from the last line to the first line (FIG. 8b)(Steps 518 and 519).

After the sheet P exits from the fixing unit 58 (guided by the fixedsheet discharge guide boards 60), and enters the switching guide board64. Since the switching guide board 64 has been switched to the (Step520), the sheet P is discharged outside the apparatus body from thedischarge roller 66 without entering the second guide member 68 (FIG.7(A)).

If there is data of a successive page (Step 521), upon completion of theprinting on both sides of the first sheet P the data of the successivepage is stored in the main buffer memory 158 (Step 522). Returning toStep 509, the microcomputer 150 starts printing the second sheet (i.e.,the data of the fourth page). This operation is thereafter repeated. Inthe absence of the data of even-numbered pages (Step 513), the printingis completed and the switching guide board 64 is down to the dischargeroller 66 side to thereby discharge the sheet (Step 523).

Although the case where one copy is made per page has been described inthe double-side printing mode of this embodiment, a plurality of copiesper page or continuous printing as shown by the flowchart of FIG. 10 maybe performed without changing the data.

That is, upon instruction of the single-side printing mode (Step 530),the microcomputer 150 sets a path for a direct output of the main buffermemory 158 to the optical beam write control mechanism 42 bypassing thedouble-side printing buffer memory 160. Upon output of the printing datafrom the host unit, the microcomputer 150 not only stores that data inthe main buffer memory 158 (Step 531) but also causes theelectrophotographic recording apparatus to get ready for printing.

That is, a sheet P of recording paper stocked in the stacker section 24or 26 disposed in the first sheet feeder 20 is introduced to the sheetfeed inlet 36 from the pickup rollers 28 or 30 the head end of the sheetP is stopped by the gate rollers 40. The sheet P is then furtherforwarded so that it will be bent, thereby causing the sheet detector 38to operate (Step 532). Accordingly, the printing process is started(Step 533). Upon completion of the printing process (Step 534), theswitching guide board 64 is down to allow the discharge roller 66 todischarge the printed sheet outside the apparatus body (Step 535). Inorder to print the same data on another sheet (Step 536), the processfrom Step 532 onward is repeated without new data transfer.

In order to print the next page after printing a predetermined number ofcopies (Step 537), the printing data for the next page gets stored inthe main buffer memory 158 (Step 538) and the process beginning withStep 532 is repeated.

On the other hand, when the double-side printing mode is selected (Step530), the microcomputer 150 sets a path for a direct output from themain buffer memory 158 to the optical beam write control mechanism 42.Upon output of printing data from the host unit, the microcomputer 150causes the main buffer memory 158 to store this printing data (Step539). When the printing data is ready, the first sheet P stocked in thestacker sections 24 or 26 is inserted by the pickup rollers 28 or 30into the sheet feed inlet 36; the head end of the sheet P is stopped bythe gate rollers 40. The sheet P is then further forwarded so that itwill be bent to operate the sheet detector 38 (Steps 540 and 541). Atthis point, the microcomputer 150 causes the data to be printed on theback of the sheet using the same process as described above (Step 542).When the printing process has been completed (Step 543), Step 544determines whether or not the data will be printed for a plurality ofcopies. If the data is to be successively printed for more pages, theswitching guide board 64 is switched to the second guide roller 74 side(Step 545). Accordingly, the sheet is forwarded upward while guided tothe sheet guide board 72 by the second guide rollers 74 bypassing thedischarge rollers 66 (FIG. 7(B)).

In parallel with the process of forwarding the printed sheet upward, themicrocomputer 150 causes the second sheet P stocked in the stackersections 24 or 26 to be discharged, returns to Step 541, and causes thesame data in the main buffer memory 158 to be printed without data beingtransferred from the host unit.

When the printing of a predetermined number of copies has been completed(Step 546) and a plurality of sheets having the same data printedthereon are accumulated on the sheet guide board 72, the microcomputer150 causes the printing data of the previous page to be transferred tothe main buffer memory 158 (Steps 547 and 548) and determines whetherthe page to be printed is odd-numbered or even-numbered (Step 540).

In parallel with this process, the first sheet P accumulated in thesheet guide board 72 is guided to the sheet take-out rollers 70 (FIG.7(C)). When it is further pulled up so that the tail end of the sheet Ppasses through the tail end detector 78, a signal is produced by thetail end detector 78 to thereby reversely rotate the take-out rollers 70(FIG. 7(D)). Accordingly, the sheet P is properly introduced into thesheet feed inlet 36 and received by the gate rollers 40. The side onwhich no data has been printed yet is positioned so that it faces thephotosensitive drum 44 which is set with a predetermined width (i.e.,providing a binding margin L that corresponds to that of the rear sideof the sheet) (FIG. 7(E)). The take-out rollers 70 are reversely rotatedso that the sheet P can be forwarded downward. This causes the sheet Pto be bent with its tail end being pressed by the gate rollers 40 whichthereby operate the sheet detector 38 (Step 549).

The microcomputer 150 starts reading the printing data of the firstpage, or the front side, from the main buffer memory 158 into thedouble-side printing buffer memory 160 with its last line as the firstpiece of data (Step 550).

The microcomputer 150 then causes the gate rollers 40 to rotatesynchronously with the writing of the data to the photosensitive drum44. The sheet is introduced to the photosensitive drum 44 "from tail tohead" in terms of the direction of printing which causes the data to beapplied to the optical beam write control mechanism 42 beginning withthe first address of the double-side printing buffer memory 160. Thus,the sheet is subjected to inverted printing from tail to head (FIG. 8B).

The sheet P that has exited from the fixing unit 58 is forwarded via thefixed sheet discharge guide boards 60 to the switching guide board 64and thereby the front-side printing is completed (Step 551). Since theswitching guide board 64 has been switched to the discharge rollers 66(Step 552), the sheet is discharged from the discharge rollers 66outside the apparatus without entering the second guide member 68 (FIG.7(A)). While other sheets remain in the sheet guide board 72 (Steps 553and 554), the microcomputer 150 returns to Step 550 to repeat theabove-described process.

If the additional pages must be printed, after completion of thedouble-side printing for a predetermined number of copies in this way(Step 555), the microcomputer 150 stores the printing data of the nexttwo pages (i.e., the fourth and third pages) in the main buffer memory158 (Step 556), and the process starting with Step 540 onward isrepeated in order to print the fourth and third pages.

If printing is terminated with an odd-numbered page (Step 544), theswitching guide board 64 is set to the discharge roller 66 side todischarge the printed sheet outside the apparatus.

When a plurality of pages are printed in this way, the sorting of theprinted sheets by page will help make the successive processes easier.

Toward this end, the page managing data, in addition to the data withinthe microcomputer 150 that is responsible for managing all of theoperations of the electrophotographic recording apparatus body 22, isapplied to the sorter 101.

The sheet discharged from the discharge rollers 66 is taken on by thesorter forwarding rollers 102, and is fed to the sorter guide boards103. If the lowermost bin tray 124 holds the first sheet and theuppermost bin tray 132 holds the fifth sheet, the bin selecting gatemember 109 is operated to thereby close the path for introducing thesheet upward when the first sheet enters the sorter guide boards 103.This maintains the horizontal position of the sheet that has enteredthrough the sorter guide boards 103, and admits the sheet into the firstbin guide 104 by the bin selecting gate member 109 to thereby introducethe sheet into the first bin tray 124 with the discharge rollers 114.Thus, the sheet discharged from the electrophotographic recordingapparatus body 22 can be subjected to a jam-free highly reliable sortingoperation because it is received in an undamaged horizontal position. Asthe sheet advances, the bin selecting gate member 109 not only closesthe bin guide 104 but also opens the path for the sorter guide boards103.

When the second sheet is successively discharged from theelectrophotographic recording apparatus body 22, the second binselecting gate member 110 is operated to close the path located over thesorter guide boards 103. This causes the sheet to pass through the firstbin selecting gate member 109 and the second bin selecting gate member110 directs the sheet to the second bin guide 105 to be discharged tothe second bin tray 126 by the discharge rollers 116. Upon completion ofthe sheet discharge operation, the bin selecting gate member 110 notonly closes the bin guide 105 but also opens the path for the sorterguide boards 103.

Likewise, by successively operating the bin selecting gate members 111and 112, the sheet direction is diverted to the bin guides 106, 107, and108 corresponding to the respective pages.

In order to discharge to the uppermost bin tray 132, which is the fifthtray, the inlets of the bin guides 104 through 107 are closed by the binselecting gates 109 through 112. Thus the sheet is guided to the binguide 108 to be discharged to the bin tray 132.

For printing large amounts of copies, the large-capacity sheet feeder140, which can accommodate 2000 sheets of recording paper in ahorizontal position, is connected to the rear side of theelectrophotographic recording apparatus body.

After the large-capacity sheet feeder 140 is attached, the microcomputer150 applies a sheet feed instruction to the pickup roller 146 of thelarge-capacity sheet feeder 140. When the pickup roller 146 startsrotating, the uppermost sheet is taken out from the large-capacitypickup pad 148 as a result of pressure from both the pickup roller 146and a plate spring. The sheet is forwarded in the direction of arrow Dthrough a guide path 31 and discharged to the sheet feed inlet 36. Whenthe sheet is further forwarded by the pickup roller 146, the head end ofthe sheet is stopped by the gate rollers 40 so that the sheet will bebent and operate the sheet detector 38.

This causes the microcomputer 150 to start the printing process which issimilar to those beginning with the sheets from the stackers 24 or 26.

After the previous sheet has been printed and output the pickup roller146 takes out the second sheet, discharges it to the sheet feed inlet36, and causes the microcomputer 150 to likewise start the printingprocess again.

In parallel with the printing process, the push-up mechanism 144 isoperated to thereby lift the stacked sheets from the bottom. Thismaintains the uppermost sheet at a predetermined height which therebyallows it to be properly taken out by the pickup roller 146.

FIG. 11 shows a second embodiment of the present invention. A recordingapparatus body 200 is of such a design that an operation panel 202, adisplay unit 204, recording cassette mounting frames 206 and the likeare vertically arranged on the sheet feeding side of the sheetforwarding path in an attempt to combine the efficient use of limitedspace and advantage of easy handling. It can be located close to apersonal computer system because it is constructed such that alloperations including recording mode selection and jam processing areperformed from the front.

The detailed structure will be illustrated with reference to FIG. 12.The apparatus body 200 is arranged so that it can be divided into twoportions, a lower unit 208 and an upper unit 210. The a sheet forwardingpath extends obliquely upward from the sheet feeding or front side (leftof the figure) to the sheet discharging, or rear side (right of thefigure). The front side of the lower unit 208 is provided with a sheetfeed unit 214 mounted so as to be separable with the vertical sheet feedguide 212.

The lower unit 208 comprises a photosensitive drum 216, a cleaning unit218, and a charger; these three components are unitized by a supportingframe (not shown) in such a manner that the photosensitive drum 216faces the sheet forwarding path and is releasably mounted. Upstream ofthe photosensitive drum 216 is a developing unit 222 which is removablymounted so that the opening of a toner hopper 220 faces the sheetforwarding path. On the rear side of the lower unit 208 are a U-shapedguide board 224 for inverting the sheet and a sorting guide board 226arranged so as to bifurcate at a sheet forwarding path switchingmechanism 228. The U-shaped guide board 224 is mounted so that its tailend is connected to an inverted sheet forwarding guide board 230 of theupper unit 210. The U-shaped guide board 224 is also arranged so that adischarge guide board 234 bifurcating upward at a sheet forwarding pathswitching mechanism 232 causes the sheet to be discharged to a dischargetray 238 through discharge rollers 236 which are arranged on thedischarge end. Alternatively, the discharge end of the sorting dischargeguide board 226 is provided with a sorting arm 242 capable of beingangled in a stepwise manner using the shaft of the first dischargeroller 240 as a pivot. This serves to discharge the sheet in any one ofa plurality of sorting trays 246 (which are releasably arranged on therear side of the lower unit 208) by the second discharge rollers 244provided on the end portion of the sorting arm 242.

On the other hand, the upper unit 210 is mounted on the lower unit 208so as to be freely opened and closed; the upper unit 210 can be invertedwith a common shaft 250 fixed on the upper portion in the front of thelower unit 208 as a pivot. On the bottom of the upper unit 210 is aguide board 252 for guiding the sheet along the sheet forwarding path.In this portion, a pair of resist rollers 254, a transfer charger 257,and a fixing unit 258 are all located above the developing unit 222 at aposition confronting the photosensitive drum 216. The upper surface ofthe upper unit 210 is entirely open and is provided with the invertedsheet forwarding guide board 230 substantially parallel with the lowerguide board 252. With the upper unit 210 closed on the lower unit 208,the end portion of the sheet forwarding path is connected to theU-shaped guide board 224 on the lower unit 208. Likewise the end portionof the downstream path is connected to the guide board 230 of a sheetforwarding mechanism 254, which is on a sheet feed unit 214 (describedlater).

Reference numeral 256 is a sheet guide member which is additionallyprovided on the upper surface of the upper unit 210. This sheet guidemember 256 is mounted so as to be freely opened and closed on the upperunit 210; thus, it can be inverted with the common shaft 250 thatsupports both the upper unit 210 and the sheet feed unit 214 (describedlater) as a pivot. The upper surface of the sheet guide member 256 isprovided with, not only the operation panel 202 which is used forspecifying various printing modes but also the display unit 204 (ifnecessary) for displaying the specified printing mode as shown in FIG.11. The lower surface of the sheet guide member 256 is provided with aguide board 258 in cooperation with the guide board 230 for guiding theinverted sheet on the upper side of the upper unit 210. A plurality ofdriven rollers 262 are paired with sheet feed roller 260 on the upperunit 210 and are supported by the shaft.

On the other hand, the sheet feed unit 214 is arranged on the front sideof the lower unit 208 and is supported so as to be pulled upward withthe common shaft 250 as a pivot; therefore, the sheet feed guide 212surface of the lower unit 208 can be opened. On the front of this sheetfeed unit 214 is provided, one or more feed cassette mounting frames 206and sheet feed rollers 266 for forwarding the sheets in the sheet feedcassettes 264. The sheet feed unit 214 has on its upper portion, theswitchback type sheet forwarding mechanism 254 for sending back into thesheet forwarding path the inverted sheet after it has been forwardedthrough the inverted sheet forwarding guide board 230 on the uppersurface of the upper unit 210. The inner surface of the sheet feed unit214 is provided with not only a sheet feed guide board 268 for guidingthe sheet forwarded by the sheet feed roller 266 in cooperation with thesheet feed guide 212 disposed in the front of the lower unit 208, butalso a guide board 274 for guiding the sheet forwarded by the sheet feedroller 272. The sheet feed roller 272 receives the sheet from alarge-capacity sheet container 270 which forms the frame for theapparatus body 200.

In FIG. 12, reference numeral 276 designates a laser writing unitprovided within the lower unit 208. Sheet feed rollers 278 are providedon the switchback type sheet forwarding mechanism 254.

Under the normal recording operation in this embodiment, the uppersurface of the obliquely opened lower unit 208 is covered by the upperunit 210, and, the opened upper surface of the upper unit 210 is coveredby the sheet guide member 256, as shown in FIG. 12. The operation panel202 located on the sheet guide member 256 is sloped on the front of theapparatus body 200 for easy operator handling (FIG. 11).

Therefore, under normal conditions, when the operator selects a desiredrecording mode and presses the start button by operating the on theoperation panel 202. The sheet in the sheet feed cassette 264 which isloaded in front of the apparatus body 200 in a manner similar to theconventional electrophotographic recording apparatus, is forwarded intothe sheet forwarding path synchronously with the writing operation. Thesheet is then subjected to the transfer and fixing operations and isfurther forwarded through the guide board 234. It is then passed throughthe bifurcated sheet inverting U-shaped guide board 224 to be dischargedto the discharge tray 238 located in the upper portion of the apparatusbody 200. When the printed sheet must be sorted, the sheet is dischargedfrom the sorting discharge guide board 226 into either one of thesorting trays 246 selected through the discharge rollers 244 on thepreviously angled sorting arm 242. When the sheet is to be printed onboth sides, the sheet inverted by the U-shaped guide board 224 isintroduced into the switchback type sheet forwarding mechanism 254through the inverted sheet forwarding guide board 230 placed on theupper surface of the upper unit 210. The sheet is then further advancedinto the sheet forwarding path from this forwarding mechanism 254 inorder to print on the rear side of the sheet.

If the sheet is jammed on its way through the sheet forwarding pathduring recording, the upper unit 210 is pulled in the direction of thearrow E with the common shaft 250 provided on the lower unit 208 tothereby invert the upper unit 210 and expose the sheet forwarding pathas shown in FIG. 13. The jammed sheet can then be removed. If the sheetis jammed on its way through the inverted sheet forwarding guide board230 at the time of double-side printing, the sheet guide member 256 ispulled in the direction of the arrow F to invert itself with the commonshaft 250 as a pivot, thereby opening the surface of the inverted sheetforwarding path on the upper unit 210 as shown in FIG. 14. The jammedsheet can then be removed. If the sheet is jammed along the sheet feedpath, the sheet feed unit 214 is rotated clockwise as indicated by thearrow G in FIG. 15, with the common shaft 250 as a pivot therebyexposing the sheet feed surface. The jammed sheet can then be removed.

In this apparatus, the opening of the upper unit 210 allows theelectrophotographic process unit to be opened concurrently. Therefore,in replenishing the toner, as shown in FIG. 13, a toner bottle (a) isinserted into the opening of the toner hopper 220 which is facing thesheet forwarding path to thereby allow the toner to be supplied to thedeveloping unit 222. In inspecting the photosensitive drum 216, thecleaning unit 218, or the charger 220, the unit integrating thesecomponents can be removed from the lower unit 208 as shown in FIG. 13.In order to inspect the transfer charger 257 and the fixing unit 258,the upper unit 210 is inverted almost 180° to position itself on thesheet feed cassette 264. This easily allows for the inspection of thesecomponents.

FIG. 16 shows a third embodiment of the electrophotographic recordingapparatus according to the present invention. This embodiment issuitable when large amounts of printing with a continuous sheet isrequired.

In the figure, reference numeral 290 designates an electrophotographicrecording apparatus body, which can roughly be divided into a lower unit292, an upper unit 294, and a stacker 296 for supplying cut sheets.

At the boundary between the lower unit 292 and the upper unit 294, aprinting path 298 formed. On the lower unit 292 are gate rollers 300 aphotosensitive drum 302, a roller 306 forming a fixing unit 304, andsheet discharge rollers 308 which are vertically aligned such that theyface the printing path 298. Around the photosensitive drum 302 are: adeveloping unit 310 equipped with a toner hopper 308 and a developingsleeve 321, a toner recovery unit 314, an eraser unit 316, and acharging unit 318.

On the other hand, the upper unit 294 is pivotably mounted on the lowerunit 292. On the surface of the lower unit 292 are connected a sheetdetector 322, the roller 324 forming the resist rollers, a transfer unit326, and the roller 305 which forms the fixing unit 304 from topdownward. Between the sheet feed inlet 328, used for receiving the sheetfrom the stacker (described later), and the discharge outlet of thefixing unit 304 is a first tractor feeder 334 arranged at an intervalcorresponding to the sheet width in the sheet forwarding direction. Thetractor feeder, stretched by rollers 330 and 332, serves to forward thesheet by inserting pins 333 into holes provided on both side edges ofthe sheet. One of the rollers 330 has a sensor 335 for detecting thephase of the first tractor feeder 334 (i.e., the position of the pins333).

On the end portion of the first tractor feeder 334 are a first guideboard 338 connected to the sheet discharge rollers 308 which isbifurcated from a first gate member 336 and a second guide board 342connected to a second gate member 340. At the tip of the second gatemember 340 are a third guide board 344 connected to a sheet tray 343containing sheets faceup and a fourth guide board 346 for forwarding theinverted sheet for double-side printing. The fourth guide board 346 isprovided with two pairs of sheet forwarding rollers 348, 350 at apredetermined interval, and the upper end of the fourth guide board 346is connected to a guide board 352 for the stacker 296.

The stacker 296 is mounted so that a second tractor feeder (describedlater) can be accommodated between itself and the lower unit 292 of theelectrophotographic recording apparatus body 200. The stacker 296 isfurther equipped with cassettes 354, 356 releasably mounted to containcut sheets in a substantially vertical position. The sheet is taken outfrom pickup rollers 358 or 360 with which the tail end of the sheetcomes in contact and is forwarded to the sheet feed inlet 328.

In the front of the stacker 296 is a guide board 352 for guiding thesheet in a substantially vertical manner. The tip of the guide board 352is provided with a tail end detector 362 for detecting the tail end ofthe sheet; on the detector 362 are rollers 364 used for introducing thesheet to the sheet feed inlet 328 from the tail end by the switchbacksystem.

In FIG. 16, the second tractor feeder 370 used for forwarding the sheetwith pins 371 is arranged so as to be stretched by the rollers 372, 374between the upper surface of lower unit 292 and the bottom surface ofthe stacker 296. The second tractor feeder 370 extends toward the frontfrom the rear of the lower unit 292 and it is in parallel with the lowerunit 292 with an interval corresponding to the sheet width. One of therollers 374 has a sensor 376 for detecting its rotational position andin cooperation with the sensor 335 of the first tractor feeder 334,detects the positional relationship between the pins 333 and 371 of thetwo tractor feeders 334, 371. On the tip of the second tractor feeder370 (i.e., on the sheet feed inlet 328 side) is a cutter 378 for cuttingthe continuous sheet, as shown in FIG. 17. At a predetermined distanceaway from the cutter 378 is a head end detector 380 for detecting thehead end of the sheet.

FIG. 17 also shows the relationship between the first tractor feeder 334and the second tractor feeder 370. Between the discharge end of thesecond tractor feeder 370 and the charge end of the first tractor feeder334 are guide boards 381 and 382 (FIG. 16). The lower guide board 381 ispivotable downward around the end portion of the second tractor feeder370 to thereby properly deliver the head end of the sheet to the firsttractor feeder 334. In FIG. 16, reference numeral 388 designates anoptical writing unit.

In order to print on a cut sheet with this embodiment, the continuoussheet is evacuated toward the second tractor feeder 370 side rather thanthe sheet feed inlet 328, and the first and second tractor feeders 334,370 must be stopped prior to operating the stacker 296. After suchpreparation, the sheet taken out by the pickup rollers 358 or 360 andenters the sheet feed inlet 328 and is stopped by the gate rollers 300,324. Upon detection of the setup of the sheet by the sheet detector 322,the sheet is forwarded to the photosensitive drum 302 by the gaterollers 300, 324, subjected to the above-described printing process, anddischarged from the fixing unit 304. At this time, if the first gatemember 336 is opened toward the first guide board 338, the sheet isforwarded from the sheet discharge roller 308 out into the apparatusbody. If a sorter 290 is connected, the sheet enters the sorter to bethereby sorted by page.

If the first gate member 336 and the second gate member 340 are openedtoward the sheet tray 343, the sheet is accommodated in the tray 343with the printed side facing upward.

In order to print on the rear side of the sheet, the second gate member340 is opened toward the fourth guide board 346, the sheet is firsttaken out into the upper end of the guide board 352 in the stacker 296,then switched back, and caused to enter the sheet feed inlet 328beginning with the tail end. Thus, the sheet is moving through theprinting path 298 with the printed side facing upward to be therebysubjected to the above-described printing process for rear-sideprinting.

The printing with the continuous sheet will next be described.

The head end of the continuous sheet P; which is folded at theperforations is inserted into the pins 371, and fixed by a pressingmember 382.

By operating the apparatus under this condition, the sheet P' isforwarded in the direction of the sheet feed inlet 328 and its head endis detected by the head end detector 380.

When the sheet P' is further forwarded between the head end detector 380and the first tractor feeder 334, the head end of the sheet P' outputfrom the second tractor feeder 370 enters the inlet of the first tractorfeeder 334 while guided by the guide board 381. The forwarding operationof the second tractor feeder 370 is adjusted based on signals from thedetectors 335 and 376 so that the holes of the sheet coincide with thepins 333, of the first tractor feeder 334. As the sheet is introducedfrom the second tractor feeder 370 in this way, the first tractor feeder334 causes the pins 333, to be inserted into the holes of the head endof the sheet for smooth receipt of the sheet. The first and secondtractor feeders 334, 370 are operated in synchronism so that, the sheetis forwarded and reaches the gate rollers 300, 324 accurately positionedby the tractor feeders 334, 370 without receiving any undesired force,and the sheet P' is then detected by the sheet detector 322. At thispoint, the gate rollers 300, 324 are operated in synchronism with thespeed of movement of the tractor feeders 334, 370; or the sheet is movedat a printing speed by the tractor feeders 334, 370 with the gaterollers 300, 324 being idle. Accordingly, the printing process asdescribed above will be performed.

When a first line of perforations reaches a predetermined region fromthe cutter 378, the second tractor feeder 370 is accelerated whilecausing the first tractor feeder 334 to continuously maintain theprinting speed. At the same time, the tip of the guide board 381 isturned downward. This causes the sheet P' present between the firsttractor feeder 334 and the second tractor feeder 370 to be slack asshown in FIG. 18. Under this condition, the first tractor feeder 334 iscausing the continuous sheet to move at the same printing speed untilthe line of perforations reaches the cutting position. When the line ofperforations reaches the cutting position, the second tractor feeder 370is stopped and the cutter 378 is operated. Thus, the sheet is cut at theperforations whereas its head end is forwarded by the first tractorfeeder 334. After the sheet has been cut, the guide board 380 isreturned to the initial position to get ready for delivery of anothersheet.

In the meantime, the sheet cut by the cutter is held by the firsttractor feeder 334 at both sides and forwarded; printing is continuouslyperformed for the remaining copies.

When the printing for one page has been completed in this way, theprinted sheet is discharged at a position determined either by the firstor second gate members 336, 340 as described previously.

In order to print the second page, the head end of the continuous sheetis delivered to the first tractor feeder 334 while the first and secondtractor feeders 334, 370 are rotated in synchronization, and theprevious printing process is repeated.

FIG. 19 shows a fourth embodiment of the present invention in whichreference numeral 390 designates an electrophotographic recordingapparatus body, which comprises, at the lower portion of the front sidethereof when installed, a sheet discharge outlet 394 for discharging asheet from sheet discharge rollers 393, and a cover member 396 thatstretches over the sheet discharge outlet 394 and can be freely openedand closed by a hinge. On the top of the apparatus body 390 is acartridge type sheet feeder 398 for containing sheets in horizontalposition. Next to the apparatus body 390 is a housing 404 that containsa control circuit board and an external storage medium reading unit.Housing 404 also has a control panel 400 consisting of operationswitches and an external storage medium inserting inlet 402 on the frontthereof.

FIG. 20 shows the internal structure of the apparatus shown in FIG. 19.In the front of the housing of the apparatus body 390 (left of thefigure) is a sheet guide board 410 in line with a sheet receiving inlet408 whose upper end is widened. Lined up from the top downward, inorder, are: a gate roller 412, a photosensitive drum 414, shield board416 (described later), and a fixing unit 418.

A magnetic brush on the surface of a magnetic sleeve 422 is in contactwith the surface of the photosensitive drum 414. A charging unit 424 isarranged at a predetermined distance in the circumferential directionwith respect to the bottom of the container of the developing unit 420.Upstream in the direction of rotation of the charging unit 424 is adischarging unit 426.

Reference numeral 428 designates an optical writing unit, disposed inthe rear space isolated by a partition wall 430. An optical beam fromthis unit is irradiated onto the surface of the photosensitive drum 414through a window 432 of the partition 430 and a clearance formed betweenthe developing unit 420 and the discharging unit 424 by an opticalsystem. The lower portion of the photosensitive drum 414 is sloped fromfront to rear, and divides the housing body space into two portions, anupper portion and a lower portion, by arranging the shield board 416integrally formed with the partition wall 430 on the back. In a spacebelow the shield board 416 are a fixing unit 418, consisting of aheating roller 434 and a pressure roller 436, and an exhaust fan 438.

On the discharge side of the fixing unit 434 is a sheet guide 440 tothereby form a sheet forwarding path for forwarding the vertical sheetto a sheet discharge outlet 394 through sheet discharge rollers 442.

Inside the cover member 396 are from top downward a sheet detector 444,a pinch roller 446 abutted against the gate roller 412, a transfer unit448 confronting the photosensitive drum 414, and a guide member 450stretching from the photosensitive drum 414 to the fixing unit 418.

Upstream of the discharge roller 392 are a tail end detector 452 and apivotably arranged sheet forwarding path selecting board 454. In thecase of double-side printing, the sheet is forwarded to a sheetinverting path 456.

In the FIG. 20, reference numeral 456 designates the sheet invertingpath formed on the cover member 396. The sheet inverting path 456 isconnected to the sheet feed inlet 408 upstream of the sheet dischargeroller 392 interposing a roller 466 for forwarding the sheet upward.

Reference numeral 398 designates the sheet feeder, which is movable onthe electrophotographic recording apparatus body 390 toward or away fromthe apparatus. The sheet feeder 398 is set to a sheet feed position (theposition indicated in FIG. 19) and is removable by pulling toward thefront. At the lower front portion of the feeder body is a sheetdischarge outlet 468 formed so that it confronts the sheet feed inlet408 of the electrophotographic recording apparatus body vertically.Above this outlet is a sheet feed inlet 470 opened for manual insertion.The sheet discharge outlet 468 is provided with not only a guide member472 that is winglike in cross-section for inverting the sheet directionby causing the horizontally forwarded sheet to be bent verticallydownward, but also a pickup roller 474 positioned to contact the headend of the sheet. Reference numeral 476 designates a spring disposed onthe bottom portion of a cassette 478 for pressing the head end of thesheet on the pickup roller 474.

In this embodiment, when the cassette 478, having the sheet P thereincontained, is set to the sheet feeder 398, the sheet is heldhorizontally with the bottom of the cassette being pushed up by thespring and the top thereof being pressed by the pickup roller 474.

Under this condition, the printing starts when a sheet is taken out fromthe cassette 478 by the pickup roller 474. The head end of the sheet isbent vertically downward to be thereby directed toward the sheetdischarge outlet 468. The sheet enters the sheet feed inlet 408 of theelectrophotographic recording apparatus body 390 and is abutted againstthe gate roller 412 by the sheet guide board 410. When the sheet isfurther pressed down by the pickup roller 474 under this condition, asignal is produced from the sheet detector 444.

Thus, the gate roller 412 is rotated to set the sheet to a referenceposition, and the sheet is forwarded further downward. The transfer unit448 transfers a toner image formed on the photosensitive drum 414 ontothe sheet. The fixing unit 418 fixes the toner on the sheet, and thesheet is further forwarded to the sheet discharge rollers 442.

The waste heat of the fixing unit 418 during this process is stoppedfrom going upward by the shield board 416 and is quickly discharged bythe exhaust fan 438 to the outside.

The toner not that has adhered to the photosensitive drum 414 but hasnot been transferred to the sheet during the developing process isscratched off by a blade located substantially below the photosensitivedrum 414 and is received by its own weight in a waste toner tank 480located right below.

In order to print on the rear side of the sheet as well, the gate member454 is switched to the sheet inversion path 456 and the sheet dischargerollers 392 are reversely rotated to thereby pull the sheet into theapparatus body side with the tail end being bit by the sheet dischargerollers 392. The tail end of the sheet is guided to the sheet inversionpath 456 by the gate member 454 then forwarded through the sheetinversion path 456 and is stopped by the gate roller 446 by entering thesheet feed inlet 408 again. Under this condition, the rear side of thesheet faces the photosensitive drum 414 so that rear-side printing isperformed by following the same process as described above.

In the case where a small number of copies are to be made using sheetsnot set in the cassette 478, the sheet is inserted into the manualinsertion sheet feed inlet 470. The sheet is then guided to the sheetdischarge outlet 468 by the guide member 472 and reaches the gate roller412 by passing through the sheet feed inlet 408. As the sheet is furtherpushed, a signal is produced by the sheet detector 444 to therebyoperate the gate roller 412. It is at this time when the hand isreleased. Then, printing is performed by the same process as describedabove.

What is claimed is:
 1. An electrophotographic recording apparatus,comprising:means for inletting a sheet into which a recording sheet isinserted; means for feeding said recording sheet to said sheet inletmeans; means, having a photosensitive drum, for forming a printing imageon said photosensitive drum based on printing data and for transferringsaid printing image onto said recording sheet; means for fixing saidtransferred printing image on said recording sheet; discharge rollermeans arranged downstream of said fixing means for discharging andrecording sheet which has been printed; means for switching thedirection of feeding said recording sheet supplied from said fixingmeans in first and second directions; means for guiding said recordingsheet having a portion thereof which is connected to said switchingmeans when said switching means switches the sheet feeding direction insaid first direction, and an outlet thereof which is located above saidsheet inlet means; means for detecting a tail end of said recordingsheet, said detecting means being arranged on an end portion of saidsheet guide means; sheet take-out roller means, located downstream ofsaid detection means, for forwarding said recording sheet, which is fedthrough said sheet guide means upward and downward by reverse rotationaccording to an output signal from said detecting means; a fixed sheetforwarding path switching member having a substantially triangularcross-section, positioned with an apex thereof adjacent to both saidsheet takeout roller means and said detecting means and configured suchthat when the sheet direction is reversed by the reverse rotation ofsaid sheet takeout means, the sheet is prevented from entering theupward sheet guide path; and reading means for writing said printingdata from a last line onward to said photosensitive drum when rear-sideprinting is performed.
 2. An apparatus as claimed in claim 1, whereinsaid sheet guide means is provided with a plurality of projectingmembers extending in the sheet forwarding direction, said plurality ofprojecting members confronting with each other staggeredly.
 3. Anapparatus as claimed in claim 1, wherein an oblique line of said sheetforwarding path switching member is provided with a guide board so as toprovide a clearance for said sheet to pass through and a direction inwhich said sheet take-out roller means are pulled is set so that itintersects said guide board on a downward side of a printing path.
 4. Anapparatus as claimed in claim 1, wherein said apex of said sheetforwarding path switching member is cut so that it can guide said sheetinto said printing path and is opened wider to accept a downward movingsheet after reversal by said sheet takeout roller means.
 5. An apparatusas claimed in claim 1, further comprising:sorter forwarding rollersprovided so as to coincide with said discharge roller means; and sortingunit for sorting said sheet discharged from said discharge roller meansinto upper and lower directions.